1. Field of the Invention
The present invention relates to an improved process for producing a formed activated coke for simultaneous desulfurization and denitrification which is useful as an adsorbent and an denitrification catalyst in a dry process for desulfurization and denitrification, as well as to a formed activated coke and a combustible gas both obtained by said process.
2. Description of the Prior Art
For reduction or removal of sulfur oxides and nitrogen oxides from various waste gases containing said oxides, there have been proposed and put into practical use a number of wet or dry processes for desulfurization and denitrification. Of these, the dry processes are advantageous in that they require no facility for waste water treatment which is complex and difficult to maintain and moreover the facility area may be small; therefore, the dry processes have drawn attention in recent years. Various adsorbents and catalysts are used in the dry processes for desulfurization and denitrification. In particular, carbonaceous adsorbents are in wide use because they are superior not only in adsorbability for sulfur oxides but also in ability as a reduction and decomposition catalyst for nitrogen oxides. As such carbonaceous adsorbents for desulfurization and denitrification, there have been used adsorbents obtained by granulating or forming raw materials composed mainly of an activated carbon. As the facilities for desulfurization and denitrification have become larger recently and a number of desulfurization and denitrification apparatuses of moving bed type or fluidized bed type have come to be adopted, the above granulated or formed activated carbons have come to create problems; that is, they have low strengths in pressure resistance, abrasion resistance, impact resistance, etc., undergo severe damage and loss during repeated use for adsorption and regeneration, and show poor economy. Various studies were made in order to eliminate the above drawbacks of activated carbon type adsorbents. As a result, there was developed a formed activated coke suitable for simultaneous desulfurization and denitrification, which is obtained by mixing a coal as primary material with various binders, etc., forming the mixture and subjecting the formed material to carbonization and activation under particular conditions (e.g. Japanese Patent Publication No. 51885/1987). The production process for this formed activated coke is greatly different from the conventional production process for activated carbon or molecular sieve carbonaceous material, which comprises forming a raw coal-binder mixture into granules, etc. and then activating the formed product. The formed activated coke is produced, for example, as follows. That is, a coal is subjected to preliminary carbonization using a carbonization retort of fluidized bed type, moving bed type or the like to obtain a semi-coke of reduced volatile matter content and high activity; this semi-coke as primary material is mixed and kneaded with secondary materials, i.e. a caking coal and a binder; the kneaded product is formed; the formed material is subjected to carbonization and activation using a rotary kiln of internal or external heating type, a vertical multi-stage retort or the like, to obtain a formed activated coke. The formed activated coke obtained, unlike the conventional activated carbon, has a uniform shape and a high strength enabling the circulation use in a reactor chamber of moving bed type. Moreover, the formed activated coke has particular pore sizes and a particular pore size distribution because it is obtained by grinding a semi-coke obtained by carbonization, forming the ground semi-coke together with a caking agent and subjecting the formed material to recarbonization; therefore, the formed activated coke can be used in simultaneous desulfurization and denitrification. The above production process for formed activated coke, however, has the following problems.
Production of semi-coke by preliminary carbonization of coal is ordinarily conducted using a carbonization retort of fluidized bed type or moving bed type. In using such a carbonization retort, a coal is subjected to carbonization in a fluidized state; therefore, the residence time of coal is not constant owing to the variation in particle size, etc. and moreover is relatively long (several minutes to several tens of minutes); the resulting semi-coke has a large variation in volatile matter content, causing a variation in quantity of final product (formed activated coke); further, fusion bonding of coal takes place in the fluidized bed or moving bed, which often causes clogging of retort. In order to take measures for these problems, it is necessary to strictly control the retort-inside temperature and oxygen concentration, which makes difficult employment of larger-sized fluidized bed or moving bed.
Activation treatment for formed coke is ordinarily conducted using, as an activation retort, a rotary kiln of internal or external heating type, an activation retort of fluidized bed type, a multi-stage retort of fluidized bed type, or the like. In these activation retort, however, the amount of formed coke fed is at best 20% or less of the retort-inside space; the reaction efficiency is low, making a large-sized retort necessary; the amount of formed coke treated is about 40 kg/hr per m.sup.3 of the retort; a large amount of energy is required in order to maintain a desired temperature, which makes it necessary to burn part of the material fed in order to maintain the reaction temperature; the distance of transfer of formed coke in retort is long in the case of a rotary kiln or a multi-stage retort, which incurs reduction in yield owing to the physical abrasion and partial combustion of material to be treated.
Further, in the activation retort, quality control is difficult because it is difficult to make constant the residence time of formed coke.
Further, in the activation retort, the gas discharged therefrom is not utilized as a combustible gas and is wasted because one gas is used as the heating gas for activation reaction and also as an activation gas, with the result that concentrations of hydrogen and carbon monoxide in the discharged gas [hydrogen and carbon monoxide are generated mainly by a reaction (C+H.sub.2 O.fwdarw.H.sub.2 +CO) in the activation reaction] are only about 10% or less.
A vertical retort of fluidized bed type is known as an activation retort into which a large amount of a carbonaceous material can be fed and which can give an activated carbon of uniform quality. This retort is suitable for production of granular activated carbon of small particle diameter (0.5-3.0 mm) or a powdered activated carbon; however, when the retort is used for production of a carbonaceous material of large particle diameter (5-30 mm) such as formed activated coke for desulfurization and denitrification, fluidization is difficult and reaction time is longer than that required in production of powdered carbon; thus, the retort has been difficult to put into practical use.